The objectives of this proposal are (1) a genetic analysis of meiotic mutants which act in male Drosophila melanogaster, and (2) the identification and localization of those regions of the chromosome that are important in chromosome pairing. This latter objective will be achieved through the analysis of the segregational patterns of various rearranged chromosomes, including inversions, translocations, compound autosomes, and Free-2-left, Free-2-right chromosomes. Four meiotic mutants are available for analysis. The properties of these mutants are different, and at least two of them permit nonhomologous pairing. Experiments have been designed to explore this behavior. Another mutant is chromosome specific and causes nondisjunction at the second meiotic division. The nature of this specificity is being examined. An analysis of the pattern of segregation in various rearranged chromosomes indicates that there are at least two different regions of the chromosome that are important for chromosome pairing. One of these regions is found in the distal third of the chromosome. These regions appear to be identical to the recomplexes found in females, and we believe they function in the initial stages of chromosome pairing in both sexes. The other region is found in the centric heterochromatin. There appear to be two sites, one on each side of the centromere. We believe the function of these sites is to properly align homologous centromeres. Homologous centromeres, once they are aligned, will then disjoin from each other, thus insuring separation of homologous chromosomes. The genetic control of meiosis is different for the two sexes. We believe that the two systems evolved from a common, primative system, and therefore, should have certain fundamental similarities. Three such possible similarities have been found.